8. Electromagnetic Waves in a Vacuum

Instructor: Wit Busza

In this session, we show how the properties (wavelength, frequency, amplitude and polarization) of an electromagnetic wave can be concluded from the equation that describes the wave and vice versa.

*NOTE: These videos were originally produced as part of a physics course that is no longer available on OCW.*

Chapters
0:00:00 Title slate
0:00:21 Electromagnetic Waves overview
0:03:29 Given the electric field of a standing EM wave, we derive the magnetic field.
0:04:54 Review of Maxwell's equations.
0:09:54 Description of a circularly polarized EM wave.
0:10:24 Similar wave but which is moving at 45 degrees to the x-axis.
0:12:54 Description of a plane polarized EM wave moving in the x-direction.
0:17:34 For the above EM standing wave, we calculate the energy density and Poynting vector.

License: Creative Commons BY-NC-SA

[quantumme]

I just had so many new insights by watching this entire video that I haven't had during the entire quarter. Thank you professor Busza, and thanks to OCW!

[lakshannavarathna]

This is by far the best explanation that Ive seen on EM wave propagation. The professor has omitted the complex math involved in derivations yet explains the gist of the concepts in depth. This is how science should be taught. Simple yet elegant.

[daedra]

Thank you very much professor. Since its midnight, I will endeavor to wake up tomorrow, safe in the knowledge that prof Wit Busza of the MIT has me covered for further studies, in the morning :D Go physics and stuff :P

[raghavmaheshwari]

sir you are a great teacher.. respect from india

[alanx]

Finally i can grasp it because it is explained very clearly. thank you.

[avonsternen]

Just as the glow of a sunny day can be reflected in a poem or painting or simply a smile, dynamic natural relations and natural principle can be reflected in mathematics. Mathematics is a language. There are other ways of communicating and understanding. :)

[arbab]

It is an interesting lecture. I would just like to add that the lecturer should have pointed the fact that standing wave does not carry energy. This is evident since the average Poyting vector is zero and so no propagation of energy. This is the very special property of the standing wave.

[tofo]

About visualizing electric fields

As an old graduate from KTH Sweden I sometimes revist the Maxwell equations and try to further understand the magic. The other day I found an interesting view on how to illustrate or understand the mysteries of electric fields when the electron starts to move.

A saw this "machine gun perspective" where you just draw the field lines out shooting off bullets in the direction of each field line and have all bullets continue in a straight line with the speed signifying speed of light - constant and uniform in all directions. (Independent of the shooters speed)

Model theory here:

I made a simple application and it actually illustrates the world quite nicely!

The electric field is all there is. Once you see the mechanics of the field in motion the rest of the fields are just perpendicular and/or circular and induced.

Interesting the field lines are straight when electron at constant speed? That is not so obvious. Notice, the faster the electron moves nearer to speed of light the E-field diverges more in the line of direction. The flow of the E-field is constant over a sphere enclosing the charge. Universe knows the charge is there granting its existence counting all the field lines all intact.

Also shaking the electron with a a nice sinusoidal motion you may think it emits waves only in that frequency, but near speed of light shaking you can see there is a broad range of frequencies emitted the closer you look in the line of motion.

My simple demo video here:

[PatrickBaptist]

So is this just a guy talking and scratching on a chalk board, didn't see any demonstration of anything, but I did just skip through the thing, not interested in theory I want to see some demonstration how radios waves can exist with a carrier that doesn't exist in a vacuum.

[boriskaragiannis]

how can nothing have a charge?...how can the "field" be made of nothing?

[robinjames]

it would have also been awesome if he could add in bells theorem.. just to really complete the picture and confuse the hell out of his students..

[LATIFAHMOHDNOR-zymq]

Page 14


They were told, "The Sun is erupting. Watch and record the storm."

[justin]

I deeply believe that the electric field and magnetic field have 90 degrees out of phase in the time axis when an electromagnetic wave is propagating (progressing).

Dipole antenna is a good way to generate and transmits electromagnetic wave. The following is my explanation, please read and think step by step carefully:
1) Electric field is generated by voltage difference between two elements while magnetic field is generated by current flows in conductor(s).
2) The voltage difference between the 2 antenna rods (or called elements) of a dipole antenna generates the electric field. Both voltage and electric field are in phase.
3) The current flows through the dipole antenna rods generates magnetic field. Both current and magnetic field are in phase.
4) The voltage difference between the 2 rods is proportional to the INTEGRATION of the current flows through the antenna rods. It works like a capacitor. That makes the voltage difference between the 2 rods 90 degrees lag the current flows in the antenna rods.
5) Combining the statements 2), 3) and 4), magnetic field leads electric field 90 degrees in time axis. NOT in phase.

If you think the phase between electric field and the magnetic field of a propagating electromagnetic wave are in phase in time axis, please point out to me which step of what I said above is incorrect.

[manosxa]

monochromatic is Greek and it does NOT mean one frequency it means μονο (mono) =single+ χρωματικ (chromatic)= coloured

[MegaZidzid]

But, I thought it will be an obvious experiment, not theories.

[aleksapjanic]

I don't really like the way he explains this. he is explaining it as though this is what is actually going on even though this is just a rough model that only works in very low energy space and in "regular" circumstances.

[mgtowvalues]

Scientism. Why not just (also) show an experiment? I though physics tested reality not just produce chalk on a board. Scientism.

[naashiirisartar]

Very bad expose, hard to follow, to avoid, bad scripting, bad elocution...fired !

[MarkMcDaniel]

He's is very lazy in explaining the formulae, and rushing through the steps.

[moukafaslouka]

Bullcrap lectures. Too much math and not enough explanations!